In order to amplify a Radio Frequency (RF) input signal, a single amplifier could be used. Such a single amplifier, however, may saturate at a certain output power level. In order to amplify the RF input signal more than this amount, two such amplifiers may be used. Each amplifier amplifies with a lower gain such that it does not saturate and so that its output signal does not exhibit unwanted distortion. The two resulting amplified signals as output by the two amplifiers are then combined to form a single higher power output signal that does not have the unwanted distortion.
FIG. 1 (Prior Art) is a diagram of a circuit 1 that includes two such amplifiers 2 and 3. The single-ended RF input signal 4 to be amplified is output by a RF input signal source 5, illustrated here in simplified symbolic form. The RF input signal 4 is supplied onto an input port 7 of a four-port passive coupler 6. The four-port 0/90 degree passive coupler 6 has four ports 7, 8, 9, and 10. The four-port passive coupler 6 splits the RF input signal 4 into a first signal 11 and a second signal 12. Port 9 is terminated by impedance 13. The second signal 12 output from port 10 is ninety degrees out of phase with respect to the first signal 11 output from port 8. The first signal 11 is supplied through a first impedance matching network 14 to the first amplifier 2. The second signal 12 is supplied through a second impedance matching network 15 to the second amplifier 3. The amplified signal as output by the second amplifier 3 is approximately ninety degrees out of phase with respect to the amplified signal as output by the first amplifier 2. A signal combiner 16 delays the amplified signal from the first amplifier 2 by ninety degrees so that it is in phase with the amplified signal as output from second amplifier 3. The signal combiner 16 combines the in-phase amplified signals so that a single high power single-ended output signal 17 is generated.
FIG. 2 (Prior Art) is a diagram of the four-port 0/90 degree passive coupler 6 of the circuit 1 of FIG. 1.
FIG. 3 (Prior Art) is a diagram of the matching network 14 of the circuit 1 of FIG. 1. The matching network 14 includes four impedances 18, 19, 20 and 21. The two matching networks 14 and 15 are of identical construction. The output impedances of the output ports of the coupler 6 are relatively small, whereas the input impedances of the amplifiers 2 and 3 are relatively large. The two output ports 8 and 9 of the coupler 6 are loaded by the matching networks 14 and 15 in a particular way so that the first and second signals 11 and 12 have substantially equal signal amplitudes and so that they have the required ninety degrees phase difference, one with respect to the other. In one example, the coupler 6 requires that all four ports be terminated with the characteristic impedance Z0 of the coupler in order for the second signal 12 as output via port 10 to have the desired ninety degree phase difference with respect to the first signal 12 as output via port 8. Accordingly, in the circuit of FIG. 1, all four ports 7-10 are terminated with load impedances of Z0.
FIG. 4 (Prior Art) is a diagram of a circuit 22 that uses two differential amplifiers 23 and 24 to amplify an RF input signal 25. Whereas the RF input signal 4 of the circuit of FIG. 1 is a single-ended signal, the RF input signal 25 of the circuit of FIG. 4 is a differential signal. A signal source 26, illustrated here in symbolic form, supplies the differential RF input signal 25 onto a differential input port 27 and 28 of a four-port 0/90 degree differential coupler 29. The four-port 0/90 degree differential coupler 29 in turn is made up of two couplers 30 and 31. The isolated port 32 and 33 of the differential coupler 29 is terminated by a terminating impedance 34. The second differential signal 35 as output from a second output port 36 and 37 of the differential coupler 29 is ninety degrees out of phase with respect to a first differential signal 38 as output from a first output port 39 and 40 of the differential coupler 29. The first differential signal 38 is supplied through a first matching network 41 to the first differential amplifier 23. The second differential signal 35 is supplied through a second matching network 42 to the second differential amplifier 24. A signal combiner 43 phase shifts the differential signal as output by the first amplifier 23 by ninety degrees and merges that phase-shifted signal with the differential signal as output by the second amplifier 24 so as to generate a single differential output signal 44.
FIG. 5 (Prior Art) is a diagram of the matching network 41 of the circuit of FIG. 4. The matching network 41 includes eight impedances 45-52. The output impedances of the two differential output ports of the differential coupler 29 are relatively small, whereas the input impedances of the two differential amplifiers 23 and 24 are relatively large. The two differential output ports of the differential coupler 29 are loaded by the matching networks 41 and 42 in a particular way so that the first and second differential signals 38 and 35 have substantially equal signal amplitudes and so that they have the required ninety degrees phase difference, one with respect to the other. In one example, the differential coupler 29 requires all four of its differential ports be terminated with the characteristic impedance Z0 in order for the second differential signal 35 as output from the second differential output port to have the desired ninety degree phase difference with respect to the first signal 38 as output from the first differential output port. Accordingly, in the circuit of FIG. 4, all four differential ports of the differential coupler 29 are terminated with load impedances of ZO.